High pressure die casting (HPDC) of engine blocks is well known in the industry. However, the HPDC process inherently yields a higher level of porosity then other metal mold casting processes, such as, low pressure die casting, squeeze casting, or semi-solid metal casting (SSM). While these other casting processes require a much higher manufacturing cost, they are often preferred over the HPDC process as the cost of repairing surface porosity defects in engine blocks offsets the low cost of the HPDC process.
The high pressure die cast process produces products with a typical porosity of 0.5 percent. Such a porosity level yields a significant amount of surface porosity defects which prevent the use of the high pressure die cast process in certain instances, especially with respect to the casting of engine blocks. For example, such surface porosity defects are not adequate for a four stroke engine block comprised of an all aluminum or hypereutectic aluminum silicon alloy that must use less than one quart of oil per 6,000 miles. In this instance, the surface porosity allows oil to penetrate past the piston ring causing “oil burning”.
The 0.5 percent porosity level is also inadequate for high pressure die cast engine blocks comprised of a hypoeutectic aluminum silicon alloy requiring chrome plating of the bores. In fact, it has been found that it is nearly impossible to chrome plate over high pressure die cast surface porosities and still have a durable chrome bore that will perform for 100,000 miles.
Further, it has been found that the O-ring seal between the combustion chamber and the water cooling chamber of an internal combustion engine is not adequately sealed if there is the typical 0.5 percent porosity resulting from the high pressure die cast process on the head deck of an engine block. This is due to the fact that the surface porosities interface with the O-ring seal. Even surface porosities as small as 0.010 inches in diameter may cause inadequate sealing.
Surprisingly, it has been found that the surface porosity problem may be remedied by a highly efficient restoration process that eliminates surface porosity on an exposed machined surface. The restoration process of the present invention allows the use of high pressure die casting in applications formerly thought to be not economically amenable to the high pressure die cast process and is superior to the restoration processes that currently define the state of the art.
In the current state of the art, surface porosity repair is achieved through the use of polymer matrix repair putty such as Devcon®, or the like. This approach to the repair of surface porosities is termed the “putty solution.” The disadvantage of the use of polymer matrix repair putty is that it may not be used on surface porosity smaller than 0.080 inches in diameter. As proper O-ring sealing requires elimination of surface porosities as small as 0.010 inches in diameter, the putty solution is an imperfect restoration process.
In order to remedy the deficiencies in the putty solution, surface porosities between 0.010 inches and 0.080 inches are drilled out so that they reach the 0.080 inch requirement and are subsequently filled with the polymer matrix repair putty. Still, the solution is imperfect as the drilling increases the surface porosity before the Devcon® repair putty is able to effectively fill the porosity with a sufficient bonding patch. The drilling step also requires the use of additional resources making the process less efficient. Additionally, the patch is not aesthestically pleasing to consumers and may convey a message that the blocks are substandard. Further, although long term life of polymer matrix repair putty patches themselves are presumed acceptable, it does decrease heat transfer locally and the long term interaction with the aluminum interfaces are in question.
Another drawback of the putty solution is that manufacturing restrictions only allow three polymer matrix repair putty patches per engine block. Only allowing three repair patches is challenging when taken in combination with high pressure die casting. The HPDC process in conjunction with the putty solution still may result in rejection rates ranging between 5 percent and 50 percent. This range of rejection rates completely disturbs plant efficiencies and productivity. The putty solution further complicates plant productivity efficiencies insofar as the putty requires curing for 24 hours before final finishing of the engine block surface. Therefore, a combination of the high pressure die cast process along with the putty solution requires a significant que for an engine block line that requires continuous seven day production to keep up with a five day demand.
An additional secondary operation utilized in the current state of the art is the use of a metal soldering patch in replacement of the polymer matrix repair putty. This “soldering solution” requires the application of a low melting point alloy on top of the identified surface porosity. Conceptually, this solution has three main advantages: 1) the patch would not be visible after cleanup; 2) it could be utilized on more than three repair sites per engine block head deck; and 3) it would not require any curing time between application and finishing, thus eliminating the need for a que of blocks.
However, the soldering solution has a major drawback in that a Galvanic couple exists between the dissimilar base metal and soldering patch. The Galvanic couple is problematic when it comes into contact with salt water, because salt water corrodes the soldering patch. As many of the engine blocks produced by this process are for marine applications, and specifically for salt water marine applications, such a problem is quite disadvantageous. Further, the soldering solution requires a heat input to the engine block surface which may result in heat distortion defects, discoloration, and overaging of the precipitation strengthened aluminum engine blocks.
As a result of the concerns about the putty solution as well as the soldering solution, alternative solutions have been explored. Surprisingly, a metal spraying restoration process captures the three noted advantages of the soldering solution without having a bonding problem, a Galvanic corrosion problem, nor a heat input problem. This novel restoration process efficiently and economically restores for use high pressure die cast engine blocks having surface porosity defects revealed by machining of the blocks. The restoration process of the current invention provides a better bond with an aluminum silicon substrate surface than either the polymer matrix repair putty or the solder material. The use of aluminum oxide grit blasting in the process further substantiates the bond. Further, there is no need to make smaller porosity defects “bigger” for applying the metal spray nor is there a limit to the number or size of porosity defects that may be repaired. Further, there is no heat distortion imparted to the engine blocks nor do Galvonic corrosion concerns exist because the spray metal applied is very similar to the substrate metal. As a result of these advantages, there is significantly less scrapping, resulting in a much more efficient production process. The restoration process of the current invention also does not require any waiting or curing time between the application of the restoration process and the final finishing of the block. Thus, no production que is needed and level loading of the blocks may be planned through the production and machining process, further increasing the efficiency of production compared to the putty solution or the soldering solution. Finally, the aesthetic problem associated with the putty solution is eliminated as repaired porosity defects are not visible after clean up.
Metal spraying ceramic materials for wear resistance has been commercialized in the enhancement of crank pin journals, as well as in metal spraying of complete cylinder bores. However, neither of the above stated uses of metal spraying have been contemplated for the restoration of surface porosity defects.
Crank pin journals are defined as the area where a connecting rod attaches to a crankshaft in an engine. The use of metal spray enhances the durability of crank pen journals to wear by building up the area of attachment. Similarly, the aircraft industry has used metal spraying of complete cylinder bores to produce a coating that reduces wear problems.
With the metal spraying being utilized to create a wear surface, the noted processes require a large capitalized systems approach. This is significantly different from the restoration of miniscule surface porosity defects to allow the use of the high pressure die casting process. Further, the process of the current invention is also quite different from any “rapid prototyping” process that builds entire articles, for the same reasons already cited. The use of this micro-area, restoration process to add value to the high pressure die cast process therefore is a new and significantly useful addition to the state of the art.